Crystal, local atomic, and local electronic structures of ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20-x}{\mathrm{Cd}}_x(0\le x\le 1.4)$: A multiband system with possible coexistence of light and heavy fermions

The partial (up to 7%) substitution of Cd for Zn in the Yb-based heavy-fermion material $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ is known to induce a slight ($\sim 20\%$) reduction of the Sommerfeld specific-heat coefficient $\gamma$ and a huge (up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient $A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio $A/{\gamma }^2$. Here, Yb $L_3$-edge x-ray absorption spectroscopy shows that the Yb valence state is close to $3+$ for all $x$, whereas x-ray diffraction reveals that Cd replaces the Zn ions only at the $16c$ site of the $Fd\overline{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn occupation. Ab initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with a substantial contribution of $\mathrm{Cd}(16c$) $5p$ levels that have weak coupling to the ${\mathrm{Yb}}^{3+}4f$ moments. These light fermions coexist with heavy fermions that originate from other conduction bands with larger participation of $\mathrm{Zn}\left(96g\right) 4p$ levels that remain strongly coupled with the ${\mathrm{Yb}}^{3+}$ local moments.

Figure 1

Left: Crystal structure of $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ with space group $Fd\overline{3}m$ (227) and lattice parameter $a=14.005\left(2\right)\AA$. Right: expanded view of the local environment of the Yb ions, with four Zn ions at the $16c$ site and 12 Zn ions at the $96g$ site.

Figure 2

Yb $L_3$-edge x-ray absorption near-edge structure (XANES) of $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ (a) and $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{18.6}{\mathrm{Cd}}_{1.4}$ (b). The closed circles represent experimental data, and the solid lines show the sum of two pseudo-Voigt peaks and a step function, where the components are shown in dotted lines. (c) Experimental XANES data for all investigated samples.

Figure 3

Fourier-transformed EXAFS data $\left|\chi \right(R\left)\right|$ (symbols) and $\mathrm{Re}\left[\chi \right(R\left)\right]$ (blue lines) for $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ (a) and $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{18.6}{\mathrm{Cd}}_{1.4}$ (b). The fitted spectra are given as red solid lines, considering the first Yb coordination shell delimited by the spectral window displayed by green lines.

Figure 4

Upper panel: calculated band dispersions of ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ (filled circles) and ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{18}{\mathrm{Cd}}_2$ (open circles). Lower panel: density of states projected into Yb $4f$, Yb $5d$, Cd $5p$, and Zn1($16c$), Zn2($48f$), and Zn3($96g$) $4p$ states for ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ ($\text{lines}+\text{symbols}$) and ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{18}{\mathrm{Cd}}_2$ (lines).